ISIS executes British aid worker David Haines; Cameron vows justice

September 13, 2014 11:21 PM

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(CNN) -- British aid worker David Haines has been executed by ISIS militants, according to a video posted Saturday to a website associated with the group, making him the third Western captive to be killed by the Islamist extremist group in recent weeks.

The ISIS video post showing Haines' beheading called his execution "a message to the allies of America."